This invention relates to a polypropylene film containing a non-migratory slip and release additive package of a synthetic or mineral wax, an aluminosilicate additive, and optionally a silicone oil and a crosslinked silicone polymer resin. This film has excellent and stable cold seal adhesive release properties and exhibits a marked improvement in stable slipperiness, excellent transparency, excellent printability, and no cold seal adhesive deadening.
A biaxially oriented polypropylene film used for cold seal release applications often performs multiple functions. It must perform in a lamination as a slip film with low COF; it must perform as a print film with good optical clarity, gloss and ink adhesion; and it must perform as a cold seal release film such that the cold seal adhesive does not stick to the film while in wound roll form.
Traditional methods for producing a functional cold seal release film have often relied upon amide-type additives. These fatty amides (e.g. erucamide, stearamide, behenamide) bloom to the surface of the film. The amide molecule has a hydrophobic and non-polar end, which tends to repel the cold seal adhesivexe2x80x94whose molecules are polar and hydrophilicxe2x80x94and thus the cold seal adhesive does not adhere strongly to the release surface containing the amides. However, the drawback of such additives is their migratory behavior in polyolefin films. The amount of additive bloom to the surface can vary with environmental conditions, and thus release properties can vary greatly depending on storage conditions seasonal changes in humidity and temperature, or geographical differences in temperature and humidity. Warmer storage conditions tend to enhance blooming of these amides, whereas cool storage conditions tend to slow the migration process. If too little additive blooms, the laminated film may xe2x80x9cblockxe2x80x9dxe2x80x94the cold seal adhesive does not release from the release film and the roll cannot be unwound. If too much additive blooms, the amides can migrate or transfer to the cold seal adhesive itselfxe2x80x94this results in xe2x80x9cdeadeningxe2x80x9d of the cold seal adhesive and a consequent loss of adhesion strength.
In addition, the use of fatty amide additives also tends to make the film hazy and less glossy. This is undesirable from the point of view of print graphics appeal. Moreover, the migratory nature of the amide additive packages make them prone to bloom to the print surface of the release film or transfer from the release surface to the print surface when in wound roll form. The presence of amides on the print surface can interfere with the wettability and adhesion of water-based inks and, especially for process print applications, cause bridging of ink dots or inconsistent dot shapes and sizes. This results in muddier colors or images and a loss of graphic appeal.
Because the cold seal release film is often used in laminations with a heat-sealable and/or barrier polyolefin film, it must also act as a slip film for good packaging machinability. The use of fatty amides are very good in providing a low COF (coefficient of friction) surface, provided they have bloomed to the surface properly. However, the variability of the slip properties of a migratory fatty amide can be highly dependent upon environmental conditions as mentioned previously. If not enough additive blooms to the slip surface, the film""s COF is higher and poor machinability can result from too much friction; if too much additive blooms, the film""s COF can be too low, resulting in poor machinability due to the film being too slippery.
An objective of this invention is to solve the aforesaid problems of conventional cold seal release films by providing an additive system that is essentially non-migratory. This will result in a cold seal release film with stable release properties, stable slip and COF properties, and no cold seal adhesive deadening. In addition, such a film will offer excellent printability and transparency.
One aspect of the present invention is a polyolefin-based laminate film comprising at least 2 layers:
a) a first polyolefin-based resin layer having a surface treated by a discharge treatment method that imparts excellent printability; and
b) a polyolefin-based mixed resin layer formed on one surface of the first polyolefin-based resin layer opposite of the surface treatment,
wherein the polyolefin-based mixed resin layer contains a first additive material in an amount of about 2-10% by weight of the polyolefin-based mixed resin layer which is at least one component selected from the group consisting of synthetic waxes or mineral-derived waxes and a second additive comprising an amount of about 0.10-0.50% by weight of the polyolefin-based mixed resin layer of an amorphous aluminosilicate, and optionally one or more component selected from the group consisting of an amount of about 0.10-0.50% by weight of the polyolefin-based mixed resin layer of a crosslinked silicone polymer and an amount of about 0.02-0.5% by weight of the polyolefin-based mixed resin layer of a silicone oil.
According to this invention, the above objective is achieved by a propylene polymer film composed of a composition comprising a 2-layer coextruded film, with the print surface modified with a discharge treatment method; and the release surface modified with a non-migratory non-polar mineral or synthetic wax additive for excellent cold seal release properties and a non-migratory silicate powder additive, and/or a crosslinked silicone polymer resin, and/or a silicone oil, for good slip and antiblock properties.
Materials which may be employed for the biaxially oriented layer are propylene homo-copolymers or copolymers of propylene and other xcex1-olefins having 2 to 10 carbon atoms. In the case of copolymers, the amount of xcex1-olefin subjected to copolymerization is less than 5% based on the weight of the copolymer as a standard. If the amount of copolymer exceeds this level, the biaxially oriented layer becomes too soft, with consequent insufficient service strength of the laminate film. Into the biaxially oriented layer various additives may be introduced (normally in the range of 0.01 to 2% based on the weight of the biaxially oriented layer as a standard). These include various additives known as additives for polypropylene, for example, stabilizers, anti-oxidants, ultra-violet absorbers, plasticizers, antistatic agents, anti-blocking agents, organic lubricants, pigments, coloring agents, nucleating agents, etc. Similarly, other kinds of polymers known as suitable for mixing into polypropylene may be added, for example, polyethylene, polybutene-1, poly(4-methylpentene-1), etc. These may be added by mixing in an amount of about 0.1 to 5% based on the weight of the biaxially oriented layer.
In a preferred embodiment, the first polyolefin-based resin layer has a thickness of about 6-40 xcexcm. In another embodiment, this polyolefin-based resin layer is made of polypropylene-based resin. The polyolefin-based mixed resin layer has a thickness of about 0.2-5.0 xcexcm.
In another embodiment, at least one component of the first additive material is a synthetic or mineral wax having a viscosity at 210xc2x0 F. of 40-150 ssu per ASTM D88, penetration at 77xc2x0 F. of 0-25 dmm per ASTM D1321, and melting point of 175-220xc2x0 F. per ASTM D127; or a synthetic or mineral wax having a viscosity at 300xc2x0 F. of 200-400 ssu per ASTM D88, penetration at 77xc2x0 F. of 0.0-25 dmm per ASTM 1321, and melting point of 200-300xc2x0 F. per ASTM D127.
In another embodiment, at least one component of the second additive material is an amorphous sodium calcium aluminosilicate having a particle size of 2-5 xcexcm and a bulk density of 0.30-0.80 g/cm3; or an amorphous aluminosilicate having a particle size of 2-5 xcexcm and a bulk density of 0.10-0.30 g/cm3.
In another embodiment, at least one component of the second additive material is a crosslinked silicone resin having a spherical average particle size of 2-5 xcexcm, specific gravity of 1.32 at 25xc2x0 F., bulk density of 0.15-0.50, and linseed oil absorption rate of 50-90 ml/100 g or a silicone oil having viscosity of 300-400 cSt., specific gravity at 77xc2x0 F. of 0.90-0.99, and volatile content of 0.001-0.005%.